Menu

Four fossil reefs done

The preservation of the coral reef fauna, which includes sea urchins, bryozoans, bivalves and gastropods, among others — and the sheer extension of the reefs here in Enriquillo — amaze us each time we walk one of these canyons.

Each canyon shares a similar aspect with the reef crest at the upper reaches, which is predominantly Acropora cervicornis (staghorn coral) in life position, i.e. it was frozen in the fossil record in the exact position it had 6,000 years ago when it was surrounded by seawater. The reef section then drops down into fore-reef habitats comprised of many coral heads dominated by Siderastrea, Diploria and lots of Orbicella. The Siderastrea and Orbicella reveal an interesting draping growth pattern (see photo) which has been suggested to have been caused by periodic high sedimentation from erosion of the mountains that surround the Lake. These sediments smothered the corals causing partial mortality.

Despite living in this environment of high sedimentation, these corals were able to not only deal with the conditions, but clearly thrive. Many of the Orbicella and Siderastrea colonies underwent hundreds of these events and continued to regenerate and build massive corals, like this example. Why did these corals thrive in these conditions while Caribbean corals fail under similar high rates of sedimentation? To me, that’s a hugely interesting question.

At each site we samples the A. cervicornis reef crest and the deeper fore-reef as distinct habitats that are easy to distinguish. So far we have sampled four sites: Cañada Honda, Las Clavellinas and Los Rios on the north of the lake, and our first site Cañon de Buho on the southern edge. We have collected 30 large bulk bags at 10kg each and 60 small bulk bags at 1kg each. The truck is getting weighed down!

The large bulk samples will be split into two size fractions – sediments greater than 2mm, which we will use to reconstruct the coral, mollusk and urchin faunas, while the sand fraction smaller than 2mm will be digested with large quantities of acetic acid, which will remove the calcium carbonate, hopefully leaving behind the shark dermal denticles which are generally lass than a millimetre in size. STRI fellow Erin Dillon will tell you more about that soon.

One of the two smaller bulk bags will be used to pick the otoliths (ear bones) of reef fish and the other will be used for the extraction of the sponge spicules and sediment analysis, to find out how big were the grains and how much organic material and carbonate they contain. With all these environmental and faunal components of the reef we hope we will be able to reconstruct many of the important aspects of these 7,000 year old Caribbean reefs to quantitively describe what “pristine” Caribbean reefs were really like.

Soon we will leave Enriquillo and start collecting sediments on modern reefs around the Dominican Republic to make a direct comparison with what once was with what exists today. I fear the modern may be somewhat depressing when we compare with what we have.